Portable communication devices, such as cellular telephones, personal digital assistants (PDAs), electronic gaming devices, laptop computers, and the like, are configured to communicate over wireless networks. Accordingly, each such portable communication device relies on a receiver and a transmitter, typically connected to a single or common antenna, for sending and receiving data and control signals over the wireless network. Of course, the receiver and transmitter may be incorporated into a transceiver, having a receiver portion and a transmitter portion. In order to use the common antenna, a duplexer is included to interface between the antenna and each of the receiver portion and the transmitter portion, so that the receiver portion is able to receive signals on a receive (downlink) frequency, and the transmitter portion is able to send transmit signals on a different transmit (uplink) frequency. The receive and transmit signals may be radio frequency (RF) signals, for example.
Various types of wireless network are implemented according to different communication standards, such as universal mobile telecommunications system (UMTS), global system for mobile communication (GSM), personal communications services (PCS), digital cellular system (DCS), international mobile telecommunication (IMT), and enhanced data rates for GSM evolution (EDGE). The communication standards identify separate bands for transmitting and receiving signals. For example, UMTS band 2 (PCS) provides an uplink frequency band of 1850 MHz-1910 MHz and a downlink frequency band of 1930 MHz-1990 MHz; UMTS band 3 (DCS) provides an uplink frequency band of 1710 MHz-1785 MHz and a downlink frequency band of 1805 MHz-1880 MHz; UMTS band 7 (IMT-E) provides an uplink frequency band of 2500 MHz-2570 MHz and a downlink frequency band of 2620 MHz-2690 MHz; and UMTS band 8 (GMS-900) provides an uplink frequency band of 880 MHz-915 MHz and a downlink frequency band of 925 MHz-960 MHz. Accordingly, a duplexer operating in compliance with a UMTS standard would include a transmit filter having a passband within the corresponding uplink frequency band, and a receive filter having a passband within the corresponding downlink frequency band.
The duplexer includes two band-pass filters having different passbands, thus preventing or reducing interference between the receive and transmit signals. That is, the duplexer includes a receive filter having a receive passband for filtering the receive signals, and a transmit filter having a transmit passband for filtering the transmit signals. The band-pass receive and transmit filters may include acoustic resonator filter elements, such as surface acoustic wave (SAW) resonators or thin film bulk acoustic resonators (FBARs), for example, for filtering the receive and transmit signals. Generally, impedance matching circuits are needed to enable the duplexer to interface with the receiver and transmitter portions of a transceiver, respectively.
One difference between a band-pass filter having SAW resonators (SAW filter) and a band-pass filter having FBARs (FBAR filter) is that the SAW filter may have an inherently differential input or output terminals for differential (balanced) signals, while the FBAR filter has inherently single-ended input and output terminals for single-ended (unbalanced) signals. Therefore, a SAW filter is able to interface directly with the receiver or transmitter portion of the transceiver, which likewise have differential inputs and output terminals. The FBAR filter, however, relies on a balun circuit (in addition to the impedance matching circuit), for example, to convert between single-ended and differential signals in order to interface with the transmitter and receiver portions of the transceiver. This adds to the size, cost and insertion loss of the component.
A matched balun circuit provides impedance matching as well as converts from single-ended input signals to differential output signals. For example, impedance of the single-ended input terminal of the matched balun must match impedance of the single-ended output terminal of the FBAR filter, and the differential impedance of the differential output terminals of the matched balun must equal some matching differential impedance of the differential input terminals of the receiver portion. The balun and impedance matching functionalities are typically provided using inductor-capacitor (LC) circuits, which require a number of inductors and capacitors, an example of which is discussed below with reference to FIG. 2. The fewer inductors and capacitors needed generally result in less space, fewer fabrication steps and lower cost and loss of the matched balun and/or the duplexer overall. Accordingly, design efforts have been made to achieve satisfactory balun and impedance matching functionality with the fewest possible LC elements. However, conventional matched baluns require a minimum of four LC components, as well as a matching circuit (e.g., an inductor) connected min series with the output terminal of the FBAR filter.